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Thunderstorm near Garajau, Madeira

Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather phenomena occur in the lowest level of the atmosphere, the troposphere, just below the stratosphere. Weather refers to day-to-day temperature and precipitation activity, whereas climate is the term for the averaging of atmospheric conditions over longer periods of time. When used without qualification, "weather" is generally understood to mean the weather of Earth.

Weather is driven by air pressure, temperature and moisture differences between one place and another. These differences can occur due to the sun's angle at any particular spot, which varies with latitude. The strong temperature contrast between polar and tropical air gives rise to the largest scale atmospheric circulations: the Hadley Cell, the Ferrel Cell, the Polar Cell, and the jet stream. Weather systems in the mid-latitudes, such as extratropical cyclones, are caused by instabilities of the jet stream flow. Because the Earth's axis is tilted relative to its orbital plane, sunlight is incident at different angles at different times of the year. On Earth's surface, temperatures usually range ±40 °C (−40 °F to 100 °F) annually. Over thousands of years, changes in Earth's orbit can affect the amount and distribution of solar energy received by the Earth, thus influencing long-term climate and global climate change.

Surface temperature differences in turn cause pressure differences. Higher altitudes are cooler than lower altitudes, as most atmospheric heating is due to contact with the Earth's surface while radiative losses to space are mostly constant. Weather forecasting is the application of science and technology to predict the state of the atmosphere for a future time and a given location. The Earth's weather system is a chaotic system; as a result, small changes to one part of the system can grow to have large effects on the system as a whole. Human attempts to control the weather have occurred throughout history, and there is evidence that human activities such as agriculture and industry have modified weather patterns.

Studying how the weather works on other planets has been helpful in understanding how weather works on Earth. A famous landmark in the Solar System, Jupiter's Great Red Spot, is an anticyclonic storm known to have existed for at least 300 years. However, weather is not limited to planetary bodies. A star's corona is constantly being lost to space, creating what is essentially a very thin atmosphere throughout the Solar System. The movement of mass ejected from the Sun is known as the solar wind.

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The water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above, and below the surface of the Earth. Since the water cycle is truly a "cycle," there is no beginning or end. Water can change states among liquid, vapor, and ice at various places in the water cycle, with these processes happening in the blink of an eye or over millions of years. Although the balance of water on Earth remains fairly constant over time, individual water molecules can come and go in a hurry.

The movement of water around, over, and through the Earth is called the water cycle.

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Did you know...

...that Hurricane Debbie is the only known tropical cyclone ever to strike Ireland?

...that the Tempest Prognosticator, one of the earliest attempts at a weather prediction device, employed live leeches in its operation?

...that eyewall replacement cycles are among the biggest challenges in forecasting tropical cyclone intensity?

...that the Braer Storm of January 1993 is likely the strongest extratropical cyclone ever recorded in the north Atlantic Ocean?

...that in medieval lore, Tempestarii are magicians with the power to control the weather?

...that the omega equation is essential to numerical weather prediction?

Recent and ongoing weather

This week in weather history...

December 3

1999: European windstorm Anatol struck the coast of Sweden, Denmark, and Germany with wind gusts of up to 175 kilometers per hour (109 mph), causing over $2 billion in damage and killing 20 people.

December 4

2001: Hurricane Olga, the largest tropical cyclone on record in the Atlantic Ocean, dissipated east of The Bahamas.

December 5

1952: The Great Smog descended on London, eventually leading to around 12,000 deaths from respiratory infections.

December 6

2003: Tropical Storm Odette, an unusual December tropical cyclone in the northern Atlantic Ocean, made landfall in Jaragua National Park, Dominican Republic. Eight people were killed due to flooding.

December 7

1991: Cyclone Val began impacting the Samoan Islands, eventually causing hundreds of millions of dollars in damage and killing 16 people.

December 8

1963: Pan Am Flight 214 was struck by lightning while in a holding pattern waiting to land in Philadelphia. The plane crashed near Elkton, Maryland, killing everyone aboard.

2007: The first in a series of winter storms began to affect portions of central and eastern North America. Over the next ten days 64 people would be killed as a result of the storms.

2011: A severe windstorm produced gusts of up to 165 miles per hour (270 km/h) in the British Isles.

December 9

2003: Tropical Storm Peter, which had been a subtropical storm, became fully tropical. With the formation of Tropical Storm Odette five days earlier, this marked the first time that two tropical cyclones formed in December since 1887.

2006: Typhoon Utor struck the Philippines with winds of more than 100 miles per hour (160 km/h).

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Sakuhei Fujiwhara (藤原咲平, Fujihara Sakuhei, October 29, 1884 – September 22, 1950) was a Japanese meteorologist who became the namesake for the Fujiwhara effect, an important interaction when forecasting cyclone tracks, particularly tropical cyclones. Novelist Jirō Nitta is his nephew and mathematician Masahiko Fujiwara is his grandnephew. He preferred to spell his surname as Fujiwhara, which is unusual by modern standards: Hepburn romanization (Fujihara or Fujiwara) is more common after World War II. Japanese texts give the furigana for his name as either Fujihara, Fujiwara or both.

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WikiProject Meteorology is a collaborative effort by dozens of Wikipedians to improve the quality of meteorology- and weather-related articles. If you would like to help, visit the project talk page, and see what needs doing.

WikiProject Severe weather is a similar project specific to articles about severe weather. Their talk page is located here.

WikiProject Tropical cyclones is a daughter project of WikiProject meteorology. The dozens of semi-active members and several full-time members focus on improving Wikipdia's coverage of tropical cyclones.

WikiProject Non-tropical storms is a collaborative project to improve articles related to winter storms, wind storms, and extratropical weather.

Wikipedia is a fully collaborative effort by volunteers. So if you see something you think you can improve, be bold and get to editing! We appreciate any help you can provide!

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